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WO2023137539A1 - Capteurs et transducteurs de pression optiques monolithiques - Google Patents

Capteurs et transducteurs de pression optiques monolithiques Download PDF

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WO2023137539A1
WO2023137539A1 PCT/CA2022/051164 CA2022051164W WO2023137539A1 WO 2023137539 A1 WO2023137539 A1 WO 2023137539A1 CA 2022051164 W CA2022051164 W CA 2022051164W WO 2023137539 A1 WO2023137539 A1 WO 2023137539A1
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thin films
multiplicity
optical
layer
multilayer stack
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Raymond DECORBY
Graham HORNIG
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means

Definitions

  • the present invention pertains to the field of pressure sensors, ultrasound detection and generation of ultrasonic waves.
  • Buckling delamination of thin films is a fairly well understood phenomenon. Within the regime of elastic deformation, the buckled areas are characterized by an increase in bending strain energy but a decrease in compressive strain energy. Buckling of a film can occur spontaneously, provided: (i) the compressive energy exceeds the bending energy for a given buckled width, and (ii) the energy release rate (per unit area under the buckle) is higher than the adhesion energy per unit area between the film (or stack of films) and its substrate. Since film delamination causes catastrophic failure of microelectronic circuits or of protective barrier coatings, buckling has traditionally been studied as a problem to be avoided.
  • delamination buckles To control the location and shape of delamination buckles, two distinct properties may be engineered: a technique for creating regions of low and high adhesion is required, and means for accurately controlling the stress within the layers to be buckled. On a laminar assembly of materials, by varying the deposition parameters, the magnitude of compressive stress for many standard thin film dielectrics can be controlled with high accuracy.
  • US Pat. No. 8,503,849 teaches an example of using this delamination and buckling process to provide for embodiments, including a guided self-assembly of straight-sided, Euler-like buckles by delamination of a multilayer stack.
  • Optical pressure sensors have well-known attributes such as immunity to electromagnetic interference (EMI) and potential for operation in harsh environments.
  • Diaphragm-based extrinsic Fabry-Perot interferometers have been amongst the most popular types.
  • a flexible membrane is configured as one mirror in a typically low-finesse, planar Fabry-Perot structure, separated from the second ‘mirror’ (often a simple optical interface such as the cleaved end facet of an optical fiber) by a sealed and ‘empty’ (typically air) cavity. Changes in external pressure deflect the membrane and modify the interference spectrum, thereby enabling optical detection.
  • Planar Fabry-Perot Interferometers are marginally unstable as optical resonators, and are thus subject to various finesse-reducing non-idealities, particularly when illuminated by non-collimated light from Gaussian laser beams or fiber modes. Accordingly, sensing is typically carried out by monitoring the shift of spectrally broad, nearly periodic Fabry-Perot fringes, which limits the detection sensitivity and/or necessitates the use of relatively complex signal processing algorithms.
  • the detection limit i.e. , the minimum resolvable shift in some measurand
  • the minimum resolvable shift in resonant wavelength ( ⁇ min) can be approximated by the linewidth ( ⁇ ); i.e. ⁇ m in ⁇ ⁇
  • ⁇ min the minimum resolvable shift in resonant wavelength
  • ⁇ the linewidth
  • s the standard deviation
  • High-frequency acoustic (ultrasound) signals are widely used for medical imaging and non-destructive testing (NDT).
  • NDT non-destructive testing
  • Piezoelectric sensors and transducers have long been the dominant commercial technology for these applications.
  • the sensitivity of piezoelectric devices scales inversely with size, which creates challenges for high-resolution imaging, especially at high acoustic frequencies, which has spurred efforts to develop alternative technologies.
  • Optical detectors for ultrasound can deliver high sensitivity in a small footprint, and are currently the subject of an intensive research effort. These devices can be categorized approximately as devices in which the refractive index of a medium is modulated (e.g. through photoelastic effects, etc.) by an incident pressure wave, and devices in which the motion of some part (e.g. a suspended membrane) is modulated by an incident pressure wave. In many cases, pressure simultaneously modulates both the refractive index and the physical dimensions, the combination producing an effective change in the optical path length.
  • optically resonant structures e.g. ring resonators or photonic crystal microcavities
  • planar Fabry-Perot (FP) etalon has played a central role, although it suffers from well-known drawbacks (e.g. limited finesse arising from beam walk-off) due to the lack of 3-dimensional light confinement. Guggenheim et al.
  • the sensor When a moving part is used to detect pressure signals, the sensor can be viewed as an optomechanical device.
  • Cavity optomechanical devices combine resonant optical structures, such as Fabry-Perot or waveguide ring resonators, with resonant mechanical structures, such as a vibrating membrane or cantilever.
  • the optical resonance is exploited to enhance the detection sensitivity while the mechanical resonance is exploited to enhance the response to force, pressure, etc.
  • the combination can enable sensitivity at the fundamental limits set by shot and thermal displacement noise.
  • many low-frequency optical pressure sensors employ a flexible membrane as part of a low-finesse planar FP cavity, the performance at ultrasound frequencies has often been hampered by the sub-optimal mechanical and optical quality of the devices.
  • NEPs as low as 0.008-0.3 mPA/Hz 1/2 for detection (in air) of ultrasound frequencies up to 1 MHz.
  • the devices cited have typically required relatively complex fabrication processes and/or inefficient and inconvenient optical coupling, involving tapered nanofibers or grating couplers.
  • the present art is in need of improved pressure detectors, capable of detecting ultrasonic pressures, as well as capable of providing ultrasonic emissions.
  • the present invention provides for a method of forming a pressure sensitive optical resonant cavity and mechanical resonator comprising forming a multilayer stack of thin films, wherein the multilayer stack of thin films provides high reflectance over some wavelength ranges of interest and is transparent over some other wavelength ranges of interest and wherein the multilayer stack of thin films acts as a barrier for fluids; embedding a patterned low-adhesion surface or layer between at least two layers within the multilayer stack of thin films; forming an evacuated or partially evacuated, enclosed optical cavity within the multilayer stack of thin films by causing delamination and buckling to occur in the regions of the patterned low- adhesion surface or layer; and wherein the buckled portion of the multilayer stack of thin films functions as a flexible membrane and mechanical resonator.
  • the optical resonance properties of the cavity are adjusted by altering the in-plane strain of the buckled portion of the multilayer.
  • changes in the in-plane strain are induced by selective absorption of light within one or more layers.
  • changes in the in-plane strain are induced by direct heating or cooling, such as by passing current through resistive heater electrodes.
  • changes in the in-plane strain are induced by applying voltage or current to one or more piezo-electric layers.
  • the low adhesion surface or layer is a fluorocarbon layer.
  • the low adhesion surface or layer is a self-assembled monolayer.
  • delamination and buckling occur spontaneously upon deposition of the multilayer stack of thin layers.
  • delamination and buckling occur through the application of mechanical vibrational energy. In another embodiment delamination and buckling occur through the introduction of a thermal cycling process. In another embodiment circular patterns are created in the low-adhesion surface or layer, such that half-symmetric or plano- concave optical resonant cavities are formed. In another embodiment a temporal modulation of the in-plane strain of the buckled portion of the multilayer induces a vibrational mechanical oscillation.
  • the present invention provides for a method for forming a multiplicity of sealed and non-sealed cavity optomechanical devices on a single wafer, comprising forming a multilayer stack of thin films, wherein the multilayer stack of thin films provides high reflectance over some wavelength ranges of interest and is transparent over some other wavelength ranges of interest and wherein the multilayer stack of thin films acts as a barrier for certain gas- and liquid-phase analytes; embedding a patterned low-adhesion surface or layer between at least two layers within the multilayer stack of thin films; forming at least one evacuated or partially evacuated, enclosed optical cavity within the multilayer stack of thin films by causing delamination and buckling to occur in the regions of the patterned low- adhesion surface or layer, and wherein the buckled portion of the multilayer stack of thin films functions as a flexible membrane and mechanical resonator; and forming at least one partially enclosed optical cavity within the multilayer stack of thin films by causing delamination and buckling to occur in the regions of
  • the optical resonance properties of the cavity are adjusted by altering the in-plane strain of the buckled portion of the multilayer.
  • changes in the in-plane strain are induced by selective absorption of light within one or more layers.
  • changes in the in-plane strain are induced by direct heating or cooling, such as by passing current through resistive heater electrodes.
  • changes in the in-plane strain are induced by applying voltage or current to one or more piezo-electric layers.
  • the low adhesion surface or layer is a fluorocarbon layer. In an alternative embodiment the low adhesion surface or layer is a self-assembled monolayer. In another embodiment delamination and buckling occur spontaneously upon deposition of the multilayer stack of thin layers. In another embodiment delamination and buckling occur through the application of mechanical vibrational energy. In another embodiment delamination and buckling occur through the introduction of a thermal cycling process. In another embodiment circular patterns are created in the low-adhesion surface or layer, such that half-symmetric or plano- concave optical resonant cavities are formed. In another embodiment temporal modulation of the in-plane strain of the buckled portion of the multilayer induces a vibrational mechanical oscillation.
  • the present invention provides for a device for detecting dynamic pressure changes in a fluid, including acoustic and ultrasound pressure changes, comprising a multiplicity of laminar thin films in which at least one of said multiplicity of laminar thin films has a patterned low adhesion surface between itself and an adjacent thin film; the multiplicity of laminar thin films selected so as to provide high optical reflectance over select wavelength ranges while being transparent to other wavelength ranges; the multiplicity of thin films act as a barrier to adjacent fluids; and an evacuated or partially evacuated, enclosed optical cavity created by delamination of at least one layer of said multilayer stack of thin film wherein the buckled portion of the multiplicity of laminar thin films results in an optically resonant cavity.
  • the low adhesion layer is a fluorocarbon layer.
  • the low adhesion layer is a self-assembled monolayer.
  • the present invention provides for a device for generating dynamic pressure changes in a fluid, including acoustic and ultrasound pressure changes, comprising a multiplicity of laminar thin films in which at least one of said multiplicity of laminar thin films has a patterned low adhesion surface between itself and an adjacent thin film; the multiplicity of laminar thin films selected so as to provide high optical reflectance over select wavelength ranges while being transparent to other wavelength ranges; the multiplicity of thin films act as a barrier to adjacent fluids; an evacuated or partially evacuated, enclosed optical cavity created by delamination of at least one layer of said multilayer stack of thin film wherein the buckled portion of the multiplicity of laminar thin films results in an optically resonant cavity; and means for adjusting the in-plane strain of the buckled portion of the multiplicity of laminar thin films.
  • the means of altering the in-plane strain of the buckled portion of the multiplicity of laminar thin films are resistive heater electrodes capable of receiving a current and in thermal communication with the buckled portion of the multiplicity of laminar thin films.
  • the means of altering the in-plane strain of the buckled portion of the multiplicity of laminar thin films are one or more piezo electric layers in mechanical communication with the buckled portion of the multiplicity of laminar thin films.
  • the present invention provides for a system for detecting dynamic pressure changes in a fluid
  • a device for generating dynamic pressure changes in a fluid including acoustic and ultrasound pressure changes
  • the device comprising a multiplicity of laminar thin films in which at least one of said multiplicity of laminar thin films has a patterned low adhesion surface between itself and an adjacent thin film, the multiplicity of laminar thin films selected so as to provide high optical reflectance over select wavelength ranges while being transparent to other wavelength ranges, the multiplicity of thin films act as a barrier to adjacent fluids, an evacuated or partially evacuated, enclosed optical cavity created by delamination of at least one layer of said multilayer stack of thin film wherein the buckled portion of the multiplicity of laminar thin films results in an optically resonant cavity, and means for adjusting the in-plane strain of the buckled portion of the multiplicity of laminar thin films; an optical emitter capable of providing an optical signal to said enclosed optical cavity within the wavelength range to which the multiplicity of laminar
  • FIG. 1 shows a schematic of a system in which the devices of the present invention were tested
  • FIG. 2 shows spectral scans observed using an exemplary Type B device of the present invention, at a series of fixed pressures
  • FIG. 3 shows a plot of the peak wavelength of the fundamental mode resonance versus pressure for an exemplary Type B device of the present invention, at a series of fixed pressures;
  • FIG. 4 shows a plot of the fundamental resonance for an exemplary Type B device of the present invention when pressure is increased over a range of values
  • FIG. 5 shows the change in peak wavelength at fixed pressures for an exemplary Type B device of the present invention
  • FIG. 6 shows spectral scans observed using an exemplary Type A device of the present invention, at a series of fixed pressures
  • FIG. 7 shows a plot of the peak wavelength of the fundamental mode resonance versus pressure for an exemplary Type A device of the present invention, at a series of fixed pressures;
  • FIG. 8 shows a vibrational spectrum for an exemplary Type B (a) and Type A (b) devices of the present invention with the fundamental resonance mode indicated;
  • FIG. 9 shows plots of predicted pressure response versus acoustic frequency, according to the single-harmonic oscillator model described herein, for Type A (a) and Type B (b) devices of the present invention
  • FIG. 10 shows a schematic illustration of acoustic (ultrasonic) signals by a buckled dome element comprising part of the devices of the present invention by way of (a) piezo-electric or resistive heater elements or (b) applying a time-varying optical signal;
  • FIG. 11 shows (a) a simple first-order thermal model for the buckled dome elements comprising part of the devices of the present invention and (b) an illustration of a strategy for engineering the thermal time constant by addition of a thermally conductive layer;
  • FIG. 12 shows a schematic of a monolithic ultrasound transducer in accordance with the present invention
  • FIG. 13 shows an illustration of a system for ultrasound detection using devices of the present invention
  • FIG. 14 shows FFT traces for various conditions using the system depicted in FIG. 13;
  • FIG. 15 shows water coupled ultrasound pulses (a,b) and the corresponding frequency-domain responses (c,d) for Type A (a,c) and Type B (b,d) devices of the present invention
  • FIG. 16 shows air coupled ultrasound pulses (a,b) and the corresponding frequency- domain responses (c,d) for Type A (a,c) and Type B (b,d) devices of the present invention
  • FIG. 17 shows the estimated NEP in air, for (a) Type A devices up to 5 MHz, and (b) Type B devices up to2 MHz.
  • US Pat. No. 8,503,849 discloses using the delamination between light guiding layers as a means to provide for substantially linear waveguides within a laminated structure.
  • the present invention provides the novel finding that the delamination, limited to a defined and constrained region within a laminar structure, which is further defined by an elastomeric boundary which itself is opposed by a light reflective surface, may act as a pressure sensor in accordance with the method and systems of the present invention, providing novel and unexpected benefits as further described herein.
  • the present invention provides for pressure sensing with on-chip buckled-dome microcavities, whose novel properties address many of the shortcomings of conventional DEFPI devices.
  • these cavities are manufactured in a completely monolithic process which can yield high-density arrays on a single chip. Additionally, the process produces inherently sealed cavities with an upper curved mirror of thickness on the order of ⁇ 1 - 2 ⁇ m. Additionally, the cavities support high-quality and stable Laguerre-Gaussian modes, naturally suited to coupling by single-mode fibers and laser beams.
  • the devices and methods of the present invention provide for an operating range and sensitivity for pressure sensing can be varied through the choice of the cavity dimensions, achieving sensitivities in the range ⁇ 0.05 - 1 nm/kPa.
  • the cavities also exhibit high finesse (>10 3 ) and high vibrational resonance frequencies (> 1 MHz), which might make them useful for sensing of low-intensity and high-speed pressure phenomena.
  • thermo-mechanical properties of dome cavities including first-order treatments of the vibrational resonance frequencies of the buckled mirror and the temperature dependence of the resonant optical modes, have been previously analyzed and are known in the art (Bitarafan, M. et al. (2015) J Opt Soc Am, B32: 1214-1220).
  • external pressure acts as a distributed force on the buckled mirror element, which modifies the height and shape of the domed cavity, and thereby modifies the optical spectrum in a manner that may be detected by means known in the art. Therefore changes in the cavity height result in a change in the optical properties of the cavity, or multiplicity of cavities, said changes in height induced primarily by changes in external pressure.
  • a set of domes with 50 ⁇ m base diameter and 5.5-period a-Si/SiO2 buckled mirrors, labeled as device Type “A” were used; as well as a set of domes with 100 pm base diameter and 4.5- period a-Si/SiO2 buckled mirrors, labelled as device Type “B”.
  • Nominally identical thin-film layers and layer thicknesses (quarter-wave layers at 1550 nm wavelength) are common to both samples.
  • nominal layer thicknesses are ⁇ 105 nm and ⁇ 265 nm for a-Si and SiO2 layers, respectively, based on typical refractive indices ( ⁇ 3.7 and ⁇ 1.46 at 1550 nm wavelength) for the sputtered films.
  • Devices of the present invention were placed inside a custom chamber as depicted in FIG. 1.
  • the sealed enclosure was plumbed to an air compressor to enable pressurization, and the pressure is monitored by a digital gauge (Baker B50015 Digital Pressure Gauge); with all pressures described herein provided as relative to the lab pressure.
  • the sample sits between input and output optical windows (High- Vacuum CF Flange Viewports for 1 .5” Windows) to enable optical transmission measurements.
  • Typical pressure-induced changes in the optical transmission spectra for a Type B dome are shown by the series of plots in FIG. 2 at a series of three fixed pressures of 0 kPa, 24 kPa and 44 kPa.
  • These Type B dome devices exhibit the characteristic resonance spectra of spherical mirror cavities, associated with stable Laguerre/Hermite-Gaussian (LG/HG) modes.
  • the fundamental transverse mode (LGoo m for a given longitudinal mode order m) is associated with the longest wavelength peak in the spectrum and the higher-order transverse modes produce a series of peaks moving towards shorter wavelengths.
  • the external pressure is increased, the cavity height is reduced and the resonance spectrum shifts towards shorter wavelengths.
  • the overall shape of the spectrum is relatively constant, with no significant change in ‘fringe contrast’ or linewidth.
  • FIG. 4 shows scans of the fundamental resonance taken at a relatively small step size (5 pm) setting of the tunable laser, and for the smallest increment in pressure ( ⁇ 300 Pa) that could be reliably controlled in the described embodiment of the system. Note that the shift in wavelength is greater than the linewidth ( ⁇ 0.2 nm), so that detection of pressure changes much smaller than 300 Pa is possible assuming sufficiently high system SNR .
  • Substrate-reflection-induced ripple is much more noticeable in these scans; partly due to the poor mode matching between the input beam spot size ( ⁇ 20 ⁇ m) and the fundamental mode spot size ( ⁇ 5 ⁇ m diameter) for these cavities.
  • the fundamental mode spot size for the larger (Type B) domes ( ⁇ 10 ⁇ m diameter) is more closely matched to the input beam, although still not perfectly.
  • the input beam spot size is determined by the long focal length lens required in this instance, in turn due to the long distance between the optical input window and the sample plane of the pressure chamber.
  • FIG. 7 provides a plot of the peak wavelength of the fundamental mode resonance versus pressure, overlaid by a linear fit of the data, for two different exemplary Type A domes.
  • the pressure response is approximately linear over the 0 - 103 kPa range, although deviations from linearity are more apparent than for the Type B domes above. This is attributed mainly to the parasitic ripple previously described, which impacts the identification of the resonance peak. While the pressure range studied was limited by the present experimental apparatus, it is contemplated that the approximately linear response extends to significantly higher pressures, given that the maximum height deflection. The maximum height deflection is derivable from the observations provided in FIG. 6 and FIG. 7, with reference to Equation 1 ;
  • S ⁇ 1 is the shift in peak wavelength of the fundamental cavity resonance, and is the longitudinal mode order of the cavity which provides for a maximum deflection of the reflective mirror of only a few nanometers (i.e. ⁇ 1 % of the starting height) at ⁇ 103 kPa.
  • the Type A domes also showed somewhat higher variation in their pressure sensitivity, for example S ⁇ ⁇ 0.055 nm/kPa and S ⁇ ⁇ 0.083 nm/kPa for the representative cavities from FIG. 7. Slight variations in buckle height mentioned above, typical for the experimental fabrication process used herein, likely have a greater impact on the residual stress (and thus stiffness) of the buckled mirror for these smaller and shorter cavities.
  • the ratio of experimental sensitivity for Type A and B domes ( S ⁇ A / S ⁇ B ⁇ 0.1 ) is in very good agreement with the predictions of a simplified theoretical model.
  • the mechanical/vibrational resonance spectra of exemplary devices of the present invention were measured using a tuned-to-slope technique as known in the art (Bitarafan, M. et al. (2015) J Opt SocAm, B32: 1214-1220, Hornig, G. et al, (2020) Opt Express, 28:28113-28125).
  • the laser is tuned to a wavelength just slightly removed from the fundamental cavity resonance (i.e. somewhere near the half-maximum transmission point of the Lorentzian cavity line-shape), such that vibrational motions of the upper mirror are translated to changes in cavity transmission.
  • the thermal vibrational frequency spectrum of the buckled mirror can thus be extracted from a Fourier transform of the time-varying intensity signal recorded by a high-speed photodetector receiver.
  • Typical results for Type B and Type A cavities are shown in FIG. 8a and FIG. 8b, respectively, with the lower SNR for the measurement of the Type A cavity due to the relatively poor input coupling efficiency.
  • the fundamental vibrational frequencies, at ⁇ 2.9 MHz for the type B cavity and at ⁇ 10.6 MHz for the Type A cavity, are in good agreement with the first- order predictions.
  • the high vibrational frequencies support the utility of devices of the present invention for sensing high frequency dynamic pressure changes, for example those associated with ultrasound waves generated as part of a signaling or as part of photoacoustic imaging.
  • ⁇ p is the induced motion in units of [m/Pa]
  • a is the base radius of the buckled dome
  • m eff , fo, and Q are the effective mass, vibrational resonance frequency, and quality factor of the mechanical oscillator, respectively.
  • fo , where keff is the effective spring constant of the buckled mirror
  • f 0w fo/(1 + ⁇ ), where fow is the resonant frequency in water.
  • the mechanical quality factor is reduced primarily through acoustic radiation into the water medium and can be approximated by Equation 3, where v w is the sound velocity in water.
  • the predicted frequency-dependent pressure responses of the Type A and Type B domes within the exemplary devices, in both air and water, are plotted in FIG. 9.
  • a peak response at the mechanical resonance frequency is predicted in each case.
  • the zero-frequency intercepts are in agreement with the static pressure sensitivities described herein, while the response at the resonant frequency is enhanced by the mechanical Q-factor.
  • the theoretical model described herein supports the exemplary devices of the present invention having significant response extending into the MHz frequency range, with the smaller Type A devices having a slightly lower response but also more gradual roll-off at higher frequencies. Extension from the theoretical modelling to the observations of the physical devices of the present invention, as described herein, higher-order mechanical resonant modes are present providing additional enhancement of the response at frequencies above the fundamental resonance.
  • Equation 4 It follows that the displacement-noise-limited NEP for an optomechanical sensor is frequency-independent (within the limits of the harmonic oscillator model). For buckled domes forming part of the devices of the present invention, by combining Equation 2 and Equation 4, new solution represented by Equation 5 arises, where NEPTD indicates noise-equivalent pressure in the thermal displacement noise limit.
  • Table 1 gives projected sensitivity limits in both air and water, for the two exemplary devices of the present invention disclosed herein. Table 1. Assumed and Predicted Parameters for Cavities in Exemplary Devices of the
  • the predicted NEPTD values are amongst the lowest reported for optical ultrasound sensors and are well corroborated by experimental disclosed further herein.
  • Reliance on a high optical Q-factor necessitates relatively sophisticated locking of the interrogation laser to the cavity resonance, while reliance on a high mechanical Q- factor can create challenges with respect to linearity and dynamic range.
  • the devices of the present invention achieve displacement-noise- limited sensitivity over a wide frequency range, despite their modest optical and mechanical Q-factors, in large part due to their highly efficient coupling between a pressure wave, the mechanical modes, and the optical mode of interest.
  • the ‘pressure participation ratio’ and ‘acousto-mechanical overlap factor’ are both very close to the ideal value of unity for the devices described herein.
  • G is large in the described herein, due to the direct correlation between mirror displacement and cavity resonance in a Fabry-Perot etalon.
  • While buckling height is typically small for domes of very small base diameter given the laminar thin films comprising the devices of the present invention, it is possible to embed solid spacer layers on top of the bottom mirror, in order to achieve an optical resonance at some desired wavelength of operation (such as the 1550 nm wavelength region employed here).
  • some desired wavelength of operation such as the 1550 nm wavelength region employed here.
  • the unique combination of bandwidth, sensitivity, and omni-directionality of the devices of the present invention are anticipated to enable new applications for air- coupled ultrasound. Further, it is contemplated that the devices of the present invention are capable of detecting MHz-range ultrasound pulses at distances in air which would be considered in the art, as extreme; providing substantial advantages.
  • the sensitivity of the devices of the present invention are contemplated as enabling high-frequency (i.e. , high resolution) air-coupled imaging and inspection, with relaxed requirements on the proximity between the sensor and the sample.
  • these devices achieve NEPs in the MHz range comparable to the noise levels (a few pPa/Hz 1/2 ) associated with professional recording studios in the 0 - 20 kHz audio band.
  • the devices since the devices have already been realized as dense on-chip arrays, they offer opportunity for spatially resolved ultrasound imaging, by way of non-limiting example, using a 2-D fiber array or a focused scanning beam configuration.
  • Buckled plates and shells offer unique options for actuation, because changes in the in-plane stress of the plate are coupled with changes in the out-of-plane deflection of the plate.
  • buckled cavities and waveguides such as those described herein, it has been previously established in the art that where is a change in the peak height of the buckled structure and is a change in the biaxial compressive stress of the buckled structure (Bitarafan, M. et al. (2015) J Opt SocAm B32:1214- 1220).
  • this enables thermal tuning of cavity resonance.
  • Tuning and actuation can also be achieved using alternative (non- thermal) means of modulating the effective in-plane stress of the buckled mirror.
  • a piezo-electric thin film e.g. a PZT or AIN film
  • a voltage to the ring-shaped piezo-electric region thus modulates the in- plane stress and the height of the buckled dome.
  • FIG. 10a shows a schematic illustration of the generation of acoustic (ultrasound) signals 1001 by a buckled dome cavity 1002, through electrical modulation of the in-plane stress of the buckled mirror.
  • the in-plane stress is modulated by applying a time-varying voltage/current to piezo-electric or resistive heater contacts 1003.
  • FIG. 10b shows a schematic illustration showing generation of acoustic signals as in FIG.
  • in-plane stress of the buckled dome is modulated by applying a time-varying optical beam.
  • An optically absorptive layer is embedded in the buckled mirror, so that time-varying optical signal by way of pulsed or modulated light induces a time-varying temperature change in the buckle, which in turn induces a time-varying out-of-plane deflection.
  • capacitive electrodes as are often employed in conventional ultrasound transducers, may be added to the buckled dome cavities in order to drive such motion.
  • those techniques typically require high-voltage electrical drive signals, especially for the relatively large spacing ( ⁇ 1 ⁇ m) typical of the mirror separation in our buckled domes.
  • ⁇ 1 ⁇ m spacing
  • Both electrical and optical signals can be used to actuate or tune the buckled dome cavities, as illustrated in FIG. 10.
  • thermo-mechanical properties of the devices While electro-thermal or photo-thermal techniques can easily be used for slow tuning of the buckled domes, actuation at MHz frequencies requires careful engineering of the thermo-mechanical properties of the devices. Specifically, the heating/cooling time-constants associated with the buckled mirror need to be on the order of the temporal period at the target vibrational frequencies. This implies that thermal time constants should be in the sub- ⁇ s range for actuation at MHz frequencies.
  • FIG. 11(a) illustrates that when thermal energy (heat) is deposited into the buckled mirrors of the devices of the present invention, the temperature of the buckled structure evolves (to first-order approximation) according to a thermal time constant T- CpIG, where Cp is the heat capacity of the buckled plate and G is a total thermal conductance between the buckled plate and its surroundings.
  • T- CpIG thermal time constant
  • Cp the heat capacity of the buckled plate
  • G is a total thermal conductance between the buckled plate and its surroundings.
  • FIG. 11b A strategy for engineering a faster and more efficient thermal response is depicted in FIG. 11b. While the specific example shown involves photo-thermal effects in an optically absorptive layer, a similar strategy could be employed by exploiting electro- thermal effect via resistive heater contacts. As shown in FIG. 11b, addition of a thin layer with high optical absorbance and high thermal conductivity 1101 can drastically alter the thermal dynamics of the buckled dome. For example, gold, copper or silver possess thermal conductivities which are several hundred times larger than the dielectric (e.g. a-Si, SiO 2 , or Ta 2 O 5 ) layers that comprise the buckled mirrors described herein. Other materials, such as graphene or diamond possess even higher thermal conductivities.
  • dielectric e.g. a-Si, SiO 2 , or Ta 2 O 5
  • a thin film of high thermal conductivity embedded within the buckled mirror can increase its effective thermal conductance by orders of magnitude while hardly changing its heat capacity.
  • a light signal 1102 which is non-resonant with the cavity modes can be used to tune or mechanically actuate the buckled dome, depositing heat within region 1103.
  • the methods and devices described herein can be used to implement an array of all- optical ultrasound transducers on a single probe unit, as shown by way of non- limiting example in FIG. 12.
  • a probe unit could incorporate buckled dome elements designed for sensing acoustic (ultrasound) signals, and other buckled dome elements designed for generation of acoustic signals. While one buckled dome element designed for acoustic generation and another element designed for acoustic detection are depicted in FIG. 12, the present invention contemplates a probe unit device which incorporates a multiplicity of elements adapted for acoustic generation and acoustic detection in an array and in equal or unequal ratios. While an all-optical cavity tuning strategy is depicted in FIG.
  • the present invention contemplates replacement or augmentation of this with electrical tuning methods, as known in the art.
  • Multiple acoustic sensing elements can be tuned into resonance with a single probe laser, through either electrical or optical means as described above.
  • the tuning signal can be light of a different wavelength than the probe laser, chosen to be non-resonant with the cavity modes and in a transparency region of the optical mirrors.
  • the tuning and probe light can be coupled to the cavity by using dichroic mirrors or fiber-optic based wavelength couplers. For example, wavelength couplers designed to efficiently combine 980 nm or 1480 nm wavelengths with 1550 nm-range wavelengths are widely available.
  • pulsed lasers operating at MHz-range repetition rates are widely available or otherwise known in the art.
  • electrical or optical means are employed to tune the individual sensing domes, it is possible to use active feedback techniques to lock the cavity resonance to a position slightly detuned from the probe laser. This would enable the use of a single stabilized probe laser to interrogate a large array of the buckled dome sensors.
  • novel devices of the present invention are particularly useful as sensing elements integrated directly in optical fiber arrays, providing particular utility as embedded sensors. Further, the novel devices of the present invention can be fabricated directly on the end of cleaved optical fibers.
  • the opportunity for beneficial optical mode matching to single-mode fibers, thereby negating the need for supplementary optics such as packaged collimators, is one particular advantage of the devices described herein.
  • the devices of present invention support stable, high-finesse cavity modes, thereby enabling employment of a range of simplified and high-accuracy detection algorithms, by way of non-limiting example peak-detection algorithms developed for Bragg grating sensors and as known in the art.
  • a system was constructed to detect ultrasound signals generated in an ambient water medium and a schematic provided as FIG. 13.
  • a chip containing an array of buckled dome cavities was glued onto a circuit board, and aligned to a pre-drilled hole in order to accommodate optical access.
  • a glass cylinder was then glued to the same board to serve as a fluid reservoir and mounted to a microscope setup as described below.
  • a tunable laser (Santec TSL-710) was coupled into a cavity under test from beneath the sample (i.e. through the silicon substrate). The laser wavelength was slightly detuned from the fundamental resonant wavelength of the cavity, so that vibrational motions of the buckled mirror are transduced into intensity variations in the light transmitted through and reflected from the cavity (i.e. the ‘tuned-to-slope technique’ known in the art).
  • Light reflected from the cavity under test was delivered to a high-speed photodetector attached to a computer for analysis purposes.
  • Light transmitted through the output window was collected by a long working distance objective lens (50x Mitutoyo Plan APO) and delivered to a near-infrared camera (Raptor Photonics Ninox 640 NX1 .7-VS-CL-640).
  • the camera was also used as the detector in obtaining spectral scans, by summing the pixel intensity over the region of the image containing the low-order cavity modes.
  • FIG. 14 shows plots of the fast-Fourier-transformed (FFT) frequency spectrum of the time-varying light reflected from the cavity.
  • FFT fast-Fourier-transformed
  • any vibrational motion of the buckled mirror i.e. due to thermal noise or driven by the ultrasound source
  • Curve 1402 in FIG. 14 shows the FFT spectrum extracted with the laser appropriately tuned and with the acoustic source off. This trace represents the PSD at the detector resulting from thermal vibrational (acousto- mechanical) noise of the buckled mirror element.
  • a fundamental mechanical resonance frequency at ⁇ 0.8 MHz was observed, in excellent agreement with the theoretical predictions for the type B dome in water. Signatures of higher-order vibrational modes are also present at higher frequency.
  • the FFT traces provide clear evidence for sensitive detection over the entire 0-40 MHz range of the photodetector receiver used.
  • the PSD was obtained for two different repetition rates of the ultrasound source, and it lies ⁇ 1-3 orders of magnitude above the thermo-mechanical noise floor over the entire range.
  • these curves were not calibrated to account for the spectral response of the ultrasound source itself, which is centered at 5 MHz and delivers lower power extending into the higher frequency range. Nevertheless, these experiments confirm that the buckled dome microcavities elements of the present invention have potential to respond to ultrasound signals extending into the tens of MHz range.
  • a plastic cylinder was glued to the same substrate to serve as a holding tank, and then filled with high purity deionized (DI) water.
  • DI deionized
  • the ultrasound transducers were placed directly overtop the device chip, at a distance corresponding to a 50 ⁇ s propagation delay in water ( ⁇ 7 cm). Ultrasound pulses were then measured and analyzed with the transducers were driven by an arbitrary function generator to enable pulses of much lower energy.
  • FIG. 15 shows a set of results for water-coupled ultrasound obtained using a 10 MHz transducer driven by a 100 mV (peak) electrical pulse with ultrasound pulses estimated to have peak-to-peak pressure of ⁇ 300 Pa.
  • the time-domain signals shown in FIG. 15a and FIG. 15c were averaged across 300 received pulses. From hydrophone measurements, the duration of the incident ultrasound pulse is on the order of a microsecond.
  • the Type A devices reproduce this pulse characteristic reasonably well, although non-periodic oscillations persist beyond the ⁇ 1 ⁇ s window, likely due to reverberations in the silicon substrate as discussed for the air case above.
  • received pulses (even for larger distances or lower pulse energies) were clearly impacted by ‘ringing’ of the mechanical resonator, due in part to their higher response and the relatively high Q-factor of their fundamental resonance in water.
  • the frequency-domain content of these pulses was analyzed by performing a DFT on a windowed portion ( ⁇ 49-51 ⁇ s) of the time-domain traces.
  • the resulting signal spectra (1501) are plotted alongside the corresponding noise spectra (1502) in FIG. 15c and FIG. 15d, for device Type A and Type B, respectively.
  • the signal trace for the Type A device clearly reflects the ⁇ 1 -16 MHz frequency content expected (from hydrophone calibrations) for the transducer used, with some resonant enhancement near 5 MHz. Consistent with the time-domain pulse, the signal spectrum for the Type B device is significantly impacted by the mechanical resonances.
  • a 3.5 MHz commercially available ultrasound pulse generator (OlympusTM 5800PR) transducer driven with a high energy (100 pj) electrical pulse.
  • the transducer-device spacing was set to ⁇ 5 mm (i.e. , ⁇ 15 ⁇ s propagation delay), and the laser was adjusted near resonance and with ⁇ 10 pW of average power received by the photodetector.
  • the corresponding frequency-domain content, shown as trace 1601 was obtained from the discrete Fourier transform (DFT) of the (300x averaged and bandpass filtered) pulse content lying between 14 ⁇ s and 16 ⁇ s.
  • DFT discrete Fourier transform
  • This signal response is plotted alongside the background thermo-mechanical noise spectrum (1602) extracted for the same cavity-laser detuning and laser power.
  • This spectrum reveals the natural vibrational modes of the buckled mirror, with a fundamental resonance frequency at ⁇ 11 MHz and a second-order resonance near ⁇ 18 MHz.
  • the shot noise spectrum (1603) extracted from a signal trace with the laser detuned from the cavity resonance and with the same average optical power as above, and the spectrum of the photodetector dark noise (1604).
  • Analogous results are also shown for a type B device (FIG. 16b and FIG. 16d), but with the transducer-device spacing set to ⁇ 7 cm (i.e., ⁇ 200 ⁇ s propagation delay) in that case.
  • the larger spacing provides preferential attenuation of the higher- frequency signal components, and thus reduced the ‘ringing’ caused by the overlap between the transducer’s spectral content and the fundamental dome resonance at ⁇ 2.4 MHz.
  • a typical time trace is shown in FIG. 16b, and the corresponding frequency- domain content is shown in FIG. 16d, where a window from 200 ⁇ s to 207 ⁇ s was used for the DFT in this case.
  • FIG. 17 shows representative plots of the extracted sensitivity for the devices.
  • Air- coupled NEP as low as ⁇ 100 and ⁇ 30 pPa/Hz 1/2 was estimated for device Type A (FIG. 17a) and Type B (FIG. 17b), respectively, in good agreement with the thermal- displacement-noise limited NEPs provided herein.
  • the shaded bands in these plots represent an approximate range of uncertainty for NEPTD from Equation 5, arising from the experimentally observed variations (over a large set of each type of device) in mechanical resonance frequency, quality factor, and effective spring constant.
  • the excellent agreement between theory and experiment, as well as the relatively flat sensitivity profile indicates that the devices are in fact operating near the mechanical-thermal noise limit.
  • the measured sensitivity is predicted to extend to very low acoustic frequencies, and this is supported by the dominance of the thermal- displacement noise floor down to frequencies in the few kHz region (not shown), below which the electronic noise begins to dominate.
  • the utility of the devices at sub- MHz frequencies was qualitatively verified including experiments at human audible frequencies ⁇ 20 kHz, in which the devices were used to receive music signals and deliver them to an audio amplifier. The small size of the devices is expected to result in a nearly omni-directional response at MHz frequencies.
  • the 3.5 MHz transducer was mounted on a rotational stage and measured the device response at various angles, and for fixed transducer-device spacing and energy of the driving pulse. An essentially non- directional response was verified in an angular range of approximately 60 degrees, and other observations suggest that this response extends to near-glancing angles.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measuring Fluid Pressure (AREA)
  • Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)
  • Investigating Or Analysing Materials By Optical Means (AREA)

Abstract

La présente invention concerne de nouveaux dispositifs permettant la détection de signaux acoustiques ou ultrasonores au sein d'un fluide au moyen d'un délaminage sélectif de parties d'une multiplicité de films laminaires, les films laminaires assurant une réflectance élevée sur des longueurs d'onde données et une transparence optique à d'autres, le délaminage conduisant à la création de chambres à résonance optique qui, lorsqu'elles sont interrogées avec un rayonnement EM, peuvent être utilisées pour réaliser une détection extrêmement sensible des signaux acoustiques ou ultrasonores.
PCT/CA2022/051164 2022-01-21 2022-07-29 Capteurs et transducteurs de pression optiques monolithiques Ceased WO2023137539A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20080030836A1 (en) * 2006-03-03 2008-02-07 Gentex Corporation Thin-film coatings, electro-optic elements and assemblies incorporating these elements
EP2064710A2 (fr) * 2006-09-06 2009-06-03 The Board of Trustees of the University of Illinois Structures à déformation contrôlée dans des interconnexions de semi-conducteurs et des nanomembranes pour dispositifs électroniques étirables
US20150277097A1 (en) * 2014-03-28 2015-10-01 Qualcomm Mems Technologies, Inc. Flexible ems device using organic materials

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20080030836A1 (en) * 2006-03-03 2008-02-07 Gentex Corporation Thin-film coatings, electro-optic elements and assemblies incorporating these elements
EP2064710A2 (fr) * 2006-09-06 2009-06-03 The Board of Trustees of the University of Illinois Structures à déformation contrôlée dans des interconnexions de semi-conducteurs et des nanomembranes pour dispositifs électroniques étirables
US20150277097A1 (en) * 2014-03-28 2015-10-01 Qualcomm Mems Technologies, Inc. Flexible ems device using organic materials

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